The field of the invention is magnetic resonance imaging methods and systems. More particularly, the invention relates to methods for MR imaging of dynamically moving objects, such as the beating heart, at a high-spatial and temporal-resolution.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant * of the nucleus). Nuclei which exhibit this phenomena are referred to herein as “spins”.
When a substance such as human tissue is subjected to a uniform, static magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal that is emitted by the excited spins after the excitation signal B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region that is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals, or “views”, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) that have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The present invention will be described in detail with reference to a variant of the Fourier transform (FT) imaging technique, which is frequently referred to as “spin-warp”. The spin-warp technique is discussed in an article entitled “Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
Object motion during the acquisition of NMR image data produces both blurring and “ghosts”. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion each view of the NMR signal is acquired in a period short enough that the object may be considered stationary during the acquisition window. In such case the blurring and ghosting is due to the inconsistent appearance of the object from view to view. Motion that changes the appearance between views such as that produced by a subject moving, by the respiration or the cardiac cycle, or by peristalsis, is referred to hereinafter as “view-to-view motion”. Motion may also change the amplitude and phase of the NMR signal as it evolves during the pulse sequence and such motion is referred to hereinafter as “in-view motion”.
Both blurring and ghosting can be reduced if the data acquisition is synchronized with the functional cycle of the object to reduce view-to-view motion. This method is known as gated NMR scanning, and its objective is to acquire NMR data at the same point during successive functional cycles so that the object “looks” the same in each view. The drawback of gating is that NMR data may be acquired only during a small fraction of the object's functional cycle, and even when the shortest acceptable pulse sequence is employed, the gating technique can significantly lengthen the data acquisition. Also, if the object motion does not repeat exactly from cycle-to-cycle, the reconstructed images represent the object time-averaged over several such cycles during which the data was acquired, rather than the object and its true dynamics. Furthermore, the imperfect cycle-to-cycle repetition results in residual view-to-view motion and image artifacts.
Another method for eliminating ghost artifacts is disclosed in U.S. Pat. No. 4,567,893, issued on Feb. 4, 1986. This prior patent teaches that the distance in the image between the ghosts and the object being imaged is maximized when the NMR pulse sequence repetition time is an odd multiple of one-fourth of the duration of the periodic signal variation. This can be used to alleviate ghosts due to respiratory motion. While this method, indeed, improves image quality, it does impose a constraint on the NMR pulse sequence repetition time and it often results in a longer total scan time. It also assumes that the motion is periodic.
Yet another method for reducing the undesirable effects due to periodic signal variations is disclosed in U.S. Pat. No. 4,706,026 issued on Nov. 10, 1987 and entitled “A Method For Reducing Image Artifacts Due To Periodic Variations In NMR Imaging.” In one embodiment of this method, an assumption is made about the signal variation period (e.g. due, for example, to subject respiration) and the view order is altered from the usual monotonically increasing phase-encoding gradient to a pre-selected view order. For a given signal variation period, a view order is chosen so as to make the NMR signal variation as a function of the phase-encoding amplitude be at a desired frequency. In one embodiment, the view order is selected such that the variation period appears to be equal to the total NMR scan time (low frequency) so that the ghost artifacts are brought as close to the object being imaged as possible. In another embodiment (high frequency), the view order is chosen to make the variation period appear to be as short as possible so as to push the ghost artifacts as far from the object as possible.
This prior method is effective in reducing artifacts, and is in some respects ideal if the variation is rather regular and at a known frequency. On the other hand, the method is not very robust if the assumption made about the motion temporal period does not hold (e.g., because the subject's breathing pattern changes or is irregular). If this occurs, the method loses some of its effectiveness because the focusing of the ghosts, either as close to the object or as far from the object as possible, becomes blurred. A solution to this problem is disclosed in U.S. Pat. No. 4,663,591 which is entitled “A Method For Reducing Image Artifacts Due To Periodic Signal Variations in NMR Imaging.” In this method, the non-monotonic view order is determined as the scan is executed and is responsive to changes in the period so as to produce a desired relationship (low frequency or high frequency) between the signal variations and the gradient parameter. The effectiveness of this method, of course, depends upon the accuracy of the means used to sense the subject motion, and particularly, any variations in the periodicity of that motion.
Imaging the heart is a particularly challenging problem. The heart itself is in motion (cardiac motion) and overlying this motion is translational, rotational and compressive motion due to breathing (respiratory motion). In certain applications, like imaging of the head or knees, it is often possible to limit the motion by asking the subject to voluntarily hold still or by using physical restraints to immobilize the part being imaged. However, for applications like cardiac imaging, it is not possible to eliminate the motion during the time period of MR data acquisition. Hence, for such applications it is necessary to develop methods for acquiring MR images in the presence of physiological motion. Also, capturing “snap-shot” images of the heart is much different than acquiring a dynamic movie of the heart. Whereas in the first case motion may be compensated for, in the second motion is to be recovered. For either goal so-called “retrospective” motion correction methods may be used after the NMR data is acquired, or “prospective” motion compensation methods may be used to adapt the MR data acquisition itself to compensate or capture the motion.
Breathholding and respiratory and cardiac gating methods are often used to reduce image artifacts in cardiac images. Such methods are effective, but they substantially increase the total scan time and with some subjects they simply cannot be used.
A number of retrospective methods have been proposed for correcting the acquired MR data to suppress the effects of subject motion. These methods typically assume a simple model for respiratory motion for example, rigid translation in 1 or 2 dimensions, rotational motion, expansion motion, etc. MR (or other sensory) “navigator” data may be acquired to estimate the respiration-induced motion parameters. As described for example, in U.S. Pat. Nos. 4,937,526 and 5,539,312, this estimate is then used to compensate for the motion during image reconstruction. These methods typically do not capture the complexity of the respiratory motion or do not accurately compensate for the complex motion.
Methods such as that disclosed in U.S. Pat. No. 6,587,707 have also been developed for prospectively correcting for motion. Such methods assume that the motion can be modeled as translation and/or rotation and then they modify the MRI pulse sequence to compensate for the estimated motion. Such models typically do not account for the considerable non-rigid deformation of the heart that occurs during respiration and residual motion artifacts occur.
As disclosed in the University of Illinois Master of Science Thesis of Qi Zhao published in 2002 and titled “Optimal Adaptive Dynamic MRI Based On time Sequential Sampling Theory,” a spatio-temporal model of cardiac motion has been developed and used to prospectively adapt the MR data acquisition k-space sampling scheme such that cardiac motion artifacts are suppressed. This technique is very effective, but it requires that k-space be sampled according to a prescribed temporal pattern and this precludes the simultaneous use of many respiratory motion artifact suppression methods, such as respiratory gating or windowing.